The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for MRI using a radial cones trajectory to sample k-space.
MRI uses the nuclear magnetic resonance phenomenon to produce images. When a substance such as human tissue is subjected to a uniform magnetic field, such as the so-called main magnetic field, B0, of an MRI system, the individual magnetic moments of the nuclei in the tissue attempt to align with this B0 field, but precess about it in random order at their characteristic Larmor frequency, ω. If the substance, or tissue, is subjected to a so-called excitation electromagnetic field, B1, that is in the plane transverse to the B0 field and that has a frequency near the Larmor frequency, the net aligned magnetic moment, referred to as longitudinal magnetization, may be rotated, or “tipped,” into the transverse plane to produce a net transverse magnetic moment, referred to as transverse magnetization. A signal is emitted by the excited nuclei or “spins,” after the excitation field, B1, is terminated, and this signal may be received and processed to form an image.
When utilizing these “MR” signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed for spatial encoding. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The measurement cycle used to acquire each MR signal is performed under the direction of a pulse sequence produced by a pulse sequencer. Clinically available MRI systems store a library of such pulse sequences, which can be prescribed to meet the needs of many different clinical applications. Research MRI systems include a library of clinically-proven pulse sequences, and also enable the development of new pulse sequences.
The MR signals acquired with an MRI system are signal samples of the subject of the examination in Fourier space, or what is often referred to in the art as “k-space.” Each MR measurement cycle, or pulse sequence, typically samples a portion of k-space along a sampling trajectory characteristic of that pulse sequence. Most pulse sequences sample k-space in a raster scan-like pattern sometimes referred to as a “spin-warp,” “Fourier,” “rectilinear,” or “Cartesian” scan. The spin-warp scan technique employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of MR spin-echo signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (“2DFT”), for example, spatial information is encoded in one direction by applying a phase encoding gradient, Gy, along that direction, and then an MR signal is acquired in the presence of a readout magnetic field gradient, Gx, in a direction orthogonal to the phase encoding direction. The readout gradient present during the MR signal acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse, Gy, is incremented, ΔGy, in the sequence of measurement cycles, or “views,” that are acquired during the scan to produce a set of k-space MR data, from which an entire image can be reconstructed.
There are many other k-space sampling patterns used by MRI systems. These include “radial,” or “projection reconstruction,” scans in which k-space is sampled as a set of radial sampling trajectories extending outward from the center of k-space, or extending from one quadrant of k-space through the center of k-space to an opposing quadrant of k-space. The pulse sequences utilized for a radial scan are typically characterized by the lack of a phase encoding gradient and the presence of a readout gradient that changes direction from one repetition of the pulse sequence to the next. There are also many k-space sampling methods that are closely related to the radial scan and that sample along a curved k-space sampling trajectory, such as a spiral trajectory, rather than a straight line radial trajectory.
Three-dimensional ultra-short echo time (“UTE”) imaging holds the potential both to visualize rapidly decaying nuclear species that would not otherwise be visible and to dramatically improve sampling efficiency. Unfortunately, achieving both of these benefits in a single scan is quite challenging. Short T2 imaging is generally performed with three-dimensional radial sampling schemes because of their robustness to signal decay. While partially compensated by compatibility with compressed sensing (“CS”), these sampling schemes are four times less efficient than Cartesian acquisitions. As a corollary, three-dimensional UTE with twisting trajectories are highly efficient with long readouts. Examples of such trajectories include cones, described by P. Irarrazabal and D. G. Nishimura in “Fast Three Dimensional Magnetic Resonance Imaging,” Magnetic Resonance in Medicine, 2005; 33(5):656-662; and Fermat looped, orthogonally encoded trajectories (“FLORET”), described by J. G. Pipe, et al., in “A new design and rationale for 3D orthogonally oversampled k-space trajectories,” Magnetic Resonance in Medicine, 2011; 66(5):1303-1311. However, these sampling schemes suffer from structured artifacts in the presence of off-resonance, data inconsistencies, or undersampling.
It would therefore be desirable to provide a method for magnetic resonance imaging that is amenable to three-dimensional UTE imaging. In particular, it would be desirable to provide a method for three-dimensional UTE imaging in which an efficient sampling scheme that is robust to image artifacts is utilized.